<template>
  <div>
      <p>num{{num}}</p>
      <p>double1 {{double1}}</p>
      <input v-model="double2">
  </div> 
</template>

<script>
export default {
    data() {
        return {
            num: 20
        }
    },
    computed: {
        double1() {
            return this.num * 2
        },
        double2: {
            get() {
                return this.num * 2
            },
            set(val) {
                this.num = val / 2;
            }
        }
    }
}
</script>

<style>

</style>